Electrical storage device and electrical storage module

ABSTRACT

An advantage of the present invention is to suppress an increase in resistance during high-rate charge and discharge and a reduction in capacity during a charge and discharge cycle. An electrical storage module of the present embodiment includes: an electrical storage device, and an elastic body that is placed with the electrical storage device and receives a load from the electrical storage device in a placement direction. The electrical storage device includes a positive electrode, a negative electrode, and a separator. The negative electrode includes a negative-electrode active material layer. The negative-electrode active material layer includes a first layer formed on the negative-electrode current collector, and a second layer that is formed on the first layer and has a higher compression modulus than the first layer. The separator has a lower compression modulus than the first layer, and the elastic body has a lower compression modulus than the separator.

CROSS REFERENCE TO RELATED APPLICATION

The entire disclosure of Japanese Patent Application No. 2020-004990filed on Jan. 16, 2020 including the specification, claims, drawings,and abstract is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to a technique of an electricalstorage device and to an electrical storage module.

BACKGROUND

An electrical storage device, e.g., a lithium-ion secondary battery, istypically provided with an electrode body in which a positive electrodeincluding a positive-electrode active material layer and a negativeelectrode including a negative-electrode active material layer arestacked with a separator interposed therebetween, and an electrolyticsolution. Such an electrical storage device is, for example, a batterythat is charged and discharged by charge carriers (e.g., lithium ions)moving between electrodes in an electrolytic solution. During the chargeof the electrical storage device, charge carriers are released from thepositive-electrode active material constituting the positive-electrodeactive material layer and are occluded into the negative-electrodeactive material constituting the negative-material active materiallayer. During the discharge, conversely, charge carriers are releasedfrom the negative-electrode active material and are occluded into thepositive-electrode active material. In this way, when charge carriersare occluded and released into and from the active materials during thecharge and discharge of the electrical storage device, the electrodebody expands and contracts.

If the expansion and contraction of the electrode body in response tothe charge and discharge of the electrical storage device presses anelectrolytic solution stored in the electrode body out of the electrodebody, unfortunately, a battery resistance may increase during high-ratecharge and discharge.

For example, JP 6587105 B proposes a secondary battery in which anegative electrode has a higher compression modulus than a separator.Furthermore, Patent Document 1 discloses that a negative electrode has ahigher compression modulus than a separator and thus the retainabilityof an electrolytic solution in an electrode body is improved and anincrease in battery resistance is suppressed particularly when chargeand discharge are repeated at a high rate.

SUMMARY Technical Problem

Since electrical storage devices tend to decrease in capacity throughrepetition of charge and discharge, it is important to suppress areduction in capacity during a charge and discharge cycle in order toextend the lives of the electrical storage devices.

It is an advantage of the present disclosure to provide an electricalstorage device and an electrical storage module that suppress anincrease in resistance during high-rate charge and discharge and areduction in capacity during a charge and discharge cycle.

Solution to Problem

An electrical storage device according to an aspect of the presentdisclosure is an electrical storage device including an electrode bodyin which a positive electrode, a negative electrode, and a separatordisposed between the positive electrode and the negative electrode arestacked, an elastic body configured to receive a load from the electrodebody in the stacking direction of the electrode body, and a housingaccommodating the electrode body and the elastic body, wherein thenegative electrode includes a negative-electrode current collector, anda negative-electrode active material layer that is formed on thenegative-electrode current collector and contains negative-electrodeactive material particles, the negative-electrode active material layerincludes a first layer formed on the negative-electrode currentcollector, and a second layer that is formed on the first layer and hasa higher compression modulus than the first layer, the separator has alower compression modulus than the first layer, and the elastic body hasa lower compression modulus than the separator.

An electrical storage module according to an aspect of the presentdisclosure is an electrical storage module including at least oneelectrical storage device, and an elastic body that is placed with theelectrical storage device and receives a load from the electricalstorage device in the placement direction of the elastic body, whereinthe electrical storage device includes an electrode body in which apositive electrode, a negative electrode, and a separator disposedbetween the positive electrode and the negative electrode are stacked,and a housing accommodating the electrode body, the negative electrodeincludes a negative-electrode current collector, and anegative-electrode active material layer that is formed on thenegative-electrode current collector and contains negative-electrodeactive material particles, the negative-electrode active material layerincluding a first layer formed on the negative-electrode currentcollector, and a second layer that is formed on the first layer and hasa higher compression modulus than the first layer, the separator has alower compression modulus than the first layer, and the elastic body hasa lower compression modulus than the separator.

Advantageous Effect of Invention

An aspect of the present disclosure can suppress an increase inresistance during high-rate charge and discharge and a reduction incapacity during a charge and discharge cycle.

BRIEF DESCRIPTION OF DRAWINGS

Embodiment(s) of the present disclosure will be described based on thefollowing figures, wherein:

FIG. 1 is a perspective view illustrating an electrical storage moduleaccording to an embodiment;

FIG. 2 is an exploded perspective view illustrating the electricalstorage module according to the embodiment;

FIG. 3 is a schematic cross-sectional view of expanding electricalstorage devices;

FIG. 4 is a schematic cross-sectional view of a negative electrode;

FIG. 5 is a schematic cross-sectional view of elastic bodies disposed ina housing;

FIG. 6 is a schematic perspective view illustrating an electrode body ofa cylindrical winding type;

FIG. 7 is a schematic perspective view illustrating an example of theelastic body; and

FIG. 8 is a partial schematic cross-sectional view of the elastic bodydisposed between the electrode body and the housing.

DESCRIPTION OF EMBODIMENT

An example of an embodiment will be specifically described below.Drawings to be referred in the description of the embodiment areschematically illustrated and thus the dimensional ratios or the like ofconstituent elements illustrated in the drawings may be different fromthose of an actual configuration.

FIG. 1 is a perspective view illustrating an electrical storage moduleaccording to the embodiment. FIG. 2 is an exploded perspective viewillustrating the electrical storage module according to the embodiment.An electrical storage module 1 includes, for example, an electricalstorage stack 2, a pair of locking members 6, and a cooling plate 8. Theelectrical storage stack 2 includes a plurality of electrical storagedevices 10, a plurality of insulating spacers 12, a plurality of elasticbodies 40, and a pair of end plates 4.

The electrical storage devices 10 are secondary batteries that can becharged and discharged, such as lithium-ion secondary batteries,nickel-metal hydride secondary batteries, or nickel-cadmium secondarybatteries. The electrical storage device 10 of the present embodiment isa so-called rectangular battery including an electrode body 38 (see FIG.3 ), an electrolytic solution, and a housing 13 shaped like a flatrectangular prism. The housing 13 includes an outer can 14 and a sealingplate 16. The outer can 14 has a substantially rectangular opening onone surface. The electrode body 38 and an electrolytic solution or thelike are stored in the outer can 14 through the opening. The outer can14 is desirably coated with an insulating film. e.g., a shrink tube,which is not illustrated. The sealing plate 16 for closing the openingand sealing the outer can 14 is provided on the opening of the outer can14. The sealing plate 16 constitutes a first surface 13 a of the housing13. The sealing plate 16 and the outer can 14 are joined to each otherby, for example, laser, friction stir welding, or brazing and soldering.

The housing 13 may be, for example, a cylindrical case or an exteriorpart composed of a laminated sheet including a metallic layer and aresin layer.

The electrode body 38 is configured such that a plurality of sheetpositive electrodes 38 a and a plurality of sheet negative electrodes 38b are alternately stacked with separators 38 d, which are interposedbetween the respective positive electrodes 38 a and negative electrodes38 b(see FIG. 3 ). The positive electrodes 38 a, the negative electrodes38 b, and the separators 38 d are stacked in a first direction X. Inother words, the first direction X is the stacking direction of theelectrode bodies 38. Moreover, the electrodes on both ends in thestacking direction face the long lateral faces of the housing 13. Thelong lateral faces will be described later. The first direction X, asecond direction Y, and a third direction Z in the drawings areorthogonal to one another.

The electrode body 38 may be an electrode body of a cylindrical windingtype, in which a strip positive electrode and a strip negative electrodeare stacked with a separator interposed therebetween, or an electrodebody of a flat winding type that is formed by flattening an electrodebody of a cylindrical winding type. For an electrode body of the flatwinding type, an outer can shaped like a rectangular prism may be used.For an electrode body of the cylindrical winding type, a cylindricalouter can is used.

On the sealing plate 16; that is, on the first surface 13 a of thehousing 13, an output terminal 18 is provided on one end in thelongitudinal direction while being electrically connected to thepositive electrode 38 a of the electrode body 38, and an output terminal18 is provided on the other end while being electrically connected tothe negative electrode 38 b of the electrode body 38. Hereinafter, theoutput terminal 18 connected to the positive electrode 38 a will bereferred to as a positive terminal 18 a, and the output terminal 18connected to the negative electrode 38 b will be referred to as anegative terminal 18 b. When it is not necessary to discriminate betweenthe polarities of the pair of the output terminals 18, the positiveterminal 18 a and the negative terminal 18 b are collectively referredto as the output terminals 18.

The outer can 14 has a bottom opposed to the sealing plate 16. The outercan 14 also has four lateral faces connecting the opening and thebottom. Two of the four lateral faces are a pair of long lateral facesconnected to two opposed long sides of the opening. The long lateralfaces are surfaces having the largest area among the faces of the outercan 14; that is, main surfaces. Moreover, the long lateral faces arelateral faces extending in a direction crossing (for example, orthogonalto) the first direction X. Two lateral faces other than the two longlateral faces are a pair of short lateral faces connected to the openingand the short sides of the bottom of the outer can 14. The bottom, longlateral faces, and short lateral faces of the outer can 14 correspondrespectively to the bottom, long lateral faces, and short lateral facesof the housing 13.

In the description of the present embodiment, for convenience, the firstsurface 13 a of the housing 13 is illustrated as the top surface of theelectrical storage device 10. Furthermore, the bottom of the housing 13corresponds to the bottom of the electrical storage device 10, the longlateral faces of the housing 13 correspond to the long lateral faces ofthe electrical storage device 10, and the short lateral faces of thehousing 13 correspond to the short lateral faces of the electricalstorage device 10. The electrical storage module 1 has a top surfacenear the top surfaces of the electrical storage devices 10, a bottomsurface near the bottoms of the electrical storage devices 10, andlateral faces near the short lateral faces of the electrical storagedevices 10. Moreover, the top surface of the electrical storage module 1is located on the upper side in a vertical direction and the bottom ofthe electrical storage module 1 is located on the lower side in thevertical direction.

The electrical storage devices 10 are provided in parallel atpredetermined intervals such that the long lateral faces of the adjacentelectrical storage devices 10 are opposed to each other. Furthermore, inthe present embodiment, the output terminals 18 of the electricalstorage devices 10 are oriented in the same direction. The outputterminals 18 may be oriented in different directions.

Two adjacent electrical storage devices 10 are arranged (placed) suchthat the positive terminal 18 a of one of the electrical storage devices10 and the negative terminal 18 b of the other electrical storage device10 are adjacent to each other. The positive terminal 18 a and thenegative terminal 18 b are connected in series via a bus bar (notillustrated). Alternatively, the output terminals 18 with the samepolarity on the adjacent electrical storage devices 10 may be connectedin parallel via a bus bar so as to form electrical-storage-deviceblocks, and the electrical-storage-device blocks may be connected inseries.

The insulating spacer 12 is disposed between two adjacent electricalstorage devices 10 and provides electrical insulation between the twoelectrical storage devices 10. The insulating spacer 12 is made of, forexample, insulating resin. The insulating spacer 12 is made of resinsincluding, for example, polypropylene, polybutylene terephthalate, andpolycarbonate. The electrical storage devices 10 and the insulatingspacers 12 are alternately placed. The insulating spacer 12 is alsodisposed between the electrical storage device 10 and the end plate 4.

The insulating spacer 12 has a flat part 20 and a wall part 22. The flatpart 20 is interposed between the opposed long lateral faces of the twoadjacent electrical storage devices 10. This ensures insulation betweenthe outer cans 14 of the adjacent electrical storage devices 10.

The wall part 22 extends from the outer edge of the flat part 20 in thedirection of arranging the electrical storage devices 10 and covers apart of the top surface, the lateral face, and a part of the bottom ofthe electrical storage device 10. This can, for example, obtain alateral-face distance between the adjacent electrical storage devices 10or between the electrical storage device 10 and the end plate 4. Thewall part 22 has a notch 24 where the bottom of the electrical storagedevice 10 is exposed. The insulating spacer 12 has urge receivingportions 26 that are placed face up at each end of the insulating spacer12 in the second direction Y.

The elastic bodies 40 are placed with the electrical storage devices 10along the first direction X. In other words, the first direction X is,as described above, the stacking direction of the electrode bodies 38and is also the placement direction of the electrical storage devices 10and the elastic bodies 40. The elastic body 40 is a sheet memberdisposed between the long lateral face of the electrical storage device10 and the flat part 20 of the insulating spacer 12, for example. Theelastic body 40 disposed between two adjacent electrical storage devices10 may be a sheet or a stack of a plurality of stacked sheets. Theelastic body 40 may be fixed to the surface of the flat part 20 withadhesive or the like. Alternatively, the flat part 20 may have arecessed portion and the elastic body 40 may be fit into the recessedportion. Furthermore, the elastic body 40 and the insulating spacer 12may be molded in one piece. Moreover, the elastic body 40 may be used asthe flat part 20. The structure and effects of the elastic body 40 willbe specifically discussed later.

The electrical storage devices 10, the insulating spacers 12, and theelastic bodies 40 that are provided in parallel are sandwiched betweenthe pair of end plates 4 in the first direction X. The end plate 4includes, for example, a metallic plate or a resin plate. The end plate4 has screw holes 4 a that penetrate the end plate 4 in the firstdirection X and into which screws 28 are to be inserted.

The pair of locking members 6 are long members that are longitudinallyextended in the first direction X. The pair of locking members 6 areopposed to each other in the second direction Y. The electrical storagestack 2 is disposed between the pair of locking members 6. The lockingmember 6 includes a body portion 30, a support portion 32, a pluralityof urging portions 34, and a pair of fixing portions 36.

The body portion 30 is a rectangular portion extending in the firstdirection X. The body portion 30 extends in parallel with the lateralfaces of the electrical storage devices 10. The support portion 32extends in the first direction X and protrudes from the lower end of thebody portion 30 in the second direction Y. The support portion 32 is acontinuous plate member that extends in the first direction X andsupports the electrical storage stack 2.

The urging portions 34 are connected to the upper end of the bodyportion 30 and protrude in the second direction Y. The support portion32 and the urging portions 34 are opposed to each other in the thirddirection Z. The urging portions 34 are placed at predeterminedintervals in the first direction X. The urging portions 34 are shapedlike, for example, leaf springs and urge the electrical storage devices10 to the support portion 32.

The pair of fixing portions 36 are plate members that protrude in thesecond direction Y from both ends of the body portion 30 in the firstdirection X. The pair of fixing portions 36 are opposed to each other inthe first direction X. The fixing portion 36 has through holes 36 awhere the screws 28 are inserted. The pair of fixing portions 36 fixesthe locking members 6 to the electrical storage stack 2.

The cooling plate 8 is a mechanism for cooling the electrical storagedevices 10. The electrical storage stack 2 locked by the pair of lockingmembers 6 is placed on the major surface of the cooling plate 8 and isfixed to the cooling plate 8 by inserting fastening members (notillustrated) such as screws into through holes 32 a of the supportportion 32 and through holes 8 a of the cooling plate 8.

FIG. 3 is a schematic cross-sectional view of the expanding electricalstorage devices. The number of electrical storage devices 10 is reducedin the illustration of FIG. 3 . Moreover, the illustration of theinternal structures of the electrical storage devices 10 is simplifiedand the insulating spacers 12 are omitted. As illustrated in FIG. 3 ,the electrode body 38 (the positive electrodes 38 a, the negativeelectrodes 38 b, the separators 38 d) is stored in each of theelectrical storage devices 10. The outer can 14 of the electricalstorage device 10 expands and contracts according to the expansion andcontraction of the electrode body 38 during charge and discharge. Theexpansion of the outer can 14 of the electrical storage device 10generates a load G1 that is applied outward in the first direction X inthe electrical storage stack 2. In other words, the elastic bodies 40placed with the electrical storage devices 10 receive loads from theelectrical storage devices 10 in the first direction X (the placementdirection of the electrical storage devices 10 and the elastic bodies40). To the electrical storage stack 2, a load G2 corresponding to theload G1 is applied by the locking member 6.

The compression moduli of the negative electrode 38 b, the separator 38d, and the elastic body 40 will be described below.

FIG. 4 is a schematic cross-sectional view of the negative electrode. Asillustrated in FIG. 4 , the negative electrode 38 b includes anegative-electrode current collector 50 and a negative-electrode activematerial layer 52 that is disposed on the negative-electrode currentcollector 50 and contains negative-electrode active material particles.The negative-electrode active material layer 52 includes a first layer52 a formed on the negative-electrode current collector 50, and a secondlayer 52 b formed on the first layer 52 a. The second layer 52 b has ahigher compression modulus than the first layer 52 a. In other words,the negative-electrode active material layer 52 has a high compressionmodulus near the surface and has a low compression modulus near thenegative-electrode current collector. Furthermore, the separator 38 dhas a lower compression modulus than the first layer 52 a. The elasticbody 40 has a lower compression modulus than the separator 38 d. Inother words, the compression modulus decreases in the order of thesecond layer 52 b near the surface>the first layer 52 a near thenegative-electrode current collector>the separator 38 d>the elastic body40. Thus, in this configuration, the second layer 52 b near the surfaceis the most resistant to deformation and the elastic body 40 is the mostdeformable. As described above, the present embodiment specifies thecompression moduli of the members, thereby suppressing an increase inresistance during high-rate charge and discharge and a reduction incapacity during a charge and discharge cycle. This mechanism is notsufficiently identified but is assumed to be configured as follows:

Typically, the electrode body 38 expands and contracts in response tothe charge and discharge of the electrical storage device 10, therebypressing an electrolytic solution in the electrode body 38 out of theelectrode body 38. In the present embodiment, however, the second layer52 b near the surface of the negative-electrode active material layer 52has a high compression modulus, thereby suppressing the expansion andcontraction of the second layer 52 b when the electrical storage device10 is charged or discharged. Furthermore, the compression modulus of thesecond layer 52 b is absorbed by the first layer 52 a having a lowercompression modulus than the second layer 52 b (in other words, thefirst layer 52 a is more deformable than the second layer 52 b).Moreover, the expansion and contraction of the first layer 52 a on thecurrent-collector side of the negative-electrode active material layer52 are absorbed by the separator 38 d having a lower compression modulusthan the first layer 52 a (in other words, the separator 38 d is moredeformable than the first layer 52 a). This effect suppresses thepressing of an electrolytic solution out of the negative-electrodeactive material layer 52 when the electrical storage device 10 ischarged or discharged. Furthermore, a stress applied to the separator 38d by the expansion and contraction of the negative-electrode activematerial layer 52 is absorbed by the elastic body 40 having a lowercompression modulus than the separator 38 d. This suppresses deformationof the separator 38 d and improves the retainability of an electrolyticsolution in the electrode body 38. Hence, an increase in resistance issuppressed during high-rate charge and discharge. In thenegative-electrode active material layer 52, the first layer 52 a nearthe negative-electrode current collector is more deformable than thesecond layer 52 b near the surface. This increases a contact area andadhesion with the negative-electrode current collector 50 and suppressesdisconnection of a conductive path from the negative-electrode currentcollector 50, thereby suppressing a reduction in capacity in a chargeand discharge cycle.

A compression modulus is calculated by dividing, when a predeterminedload is applied to a sample in the thickness direction, the deformationamount of the sample in the thickness direction by a compression areaand then multiplying the deformation amount by the thickness of thesample as expressed by the following formula: Compression modulus(MPa)=load (N)/compression area (mm²)/sample deformation amount (mm) xsample thickness (mm). In the measurement of the compression modulus ofthe negative-electrode active material layer 52, the compression modulusof the negative-electrode current collector 50 is measured, thecompression modulus of sample 1 in which the first layer 52 a is formedon the negative-electrode current collector 50 is measured, and thecompression modulus of sample 2 in which the second layer 52 b is formedon the first layer 52 a on the negative-electrode current collector 50is measured. Based on the compression moduli of the negative-electrodecurrent collector 50 and sample 1, the compression modulus of the firstlayer 52 a is calculated. Based on the compression moduli of sample 1and sample 2, the compression modulus of the second layer 52 b iscalculated. When the compression moduli of the first layer 52 a and thesecond layer 52 b are determined from the produced negative electrode 38b, the compression modulus of the negative electrode 38 b is measured,the compression modulus of sample 1 in which the second layer 52 b isremoved from the negative electrode is measured, and then thecompression modulus of the negative-electrode current collector 50 ismeasured. Based on the measured compression moduli, the compressionmoduli of the first layer 52 a and the second layer 52 b can bedetermined.

FIG. 5 is a schematic cross-sectional view of the elastic bodiesdisposed in the housing. When the elastic bodies 40 are placed with theelectrical storage devices 10 as described above, the elastic bodies 40are not always placed outside the housing 13. The elastic bodies 40 maybe placed inside the housing 13. The elastic bodies 40 in FIG. 5 aredisposed on both ends of the electrode body 38 in the stacking direction(first direction X) of the electrode bodies 38. The elastic body 40 isinterposed between the inner wall of the housing 13 and the electrodebody 38.

When the electrode body 38 is expanded by, for example, the charge anddischarge of the electrical storage device 10, a load applied outward inthe first direction X is generated in the electrode body 38. In otherwords, the elastic bodies 40 placed in the housing 13 receive a loadfrom the electrode body 38 in the first direction X (the stackingdirection of the electrode body). The same effect can be obtained if thecompression modulus satisfies the relationship of the first layer 52a>the second layer 52 b>the separator 38 d>the elastic body 40.

The elastic bodies 40 in the housing 13 may be placed at any positionsso long as a load can be received from the electrode body 38 in thestacking direction of the electrode body 38. For example, the elasticbody 40 may be placed in a winding core 39 of the electrode body 38 ofthe cylindrical winding type illustrated in FIG. 6 . The stackingdirection of the electrode body 38 of the cylindrical winding type is aradial direction (R) of the electrode body 38. As the electrode body 38expands or contracts, a load is generated in the electrode body 38 inthe stacking direction (the radial direction (R) of the electrode body38) of the electrode body 38, and the elastic body 40 in the windingcore 39 receives the load in the stacking direction of the electrodebody 38. If the multiple electrode bodies 38 are placed in the housing13, which is not illustrated, the elastic body 40 may be disposedbetween the adjacent electrode bodies 38. Also in the case of the flatwinding type, the electrode body may be placed at the center of theelectrode body.

The negative electrode 38 b can be produced by using, for example, afirst negative-electrode mixture slurry containing negative-electrodeactive material particles P1 and a binder, and a secondnegative-electrode mixture slurry containing negative-electrode activematerial particles P2 and a binder. Specifically, the surface of thenegative-electrode current collector 50 is coated with the firstnegative-electrode mixture slurry, and the coating is dried. Thereafter,the second negative-electrode mixture slurry is applied onto a firstcoating formed by the first negative-electrode mixture shiny, and asecond coating is dried, thereby obtaining the negative electrode 38 bin which the negative-electrode active material layer 52 including thefirst layer 52 a and the second layer 52 b is formed on thenegative-electrode current collector 50. A method of adjusting thecompression moduli of the first layer 52 a and the second layer 52 b is,for example, a method of rolling the formed first coating and the formedsecond coating and adjusting the roll forces of the coatings.Alternatively, the compression moduli can be also adjusted by changing,for example, the material properties and physical properties ofnegative-electrode active materials used for the first layer 52 a andthe second layer 52 b. The adjustment of compression moduli of thelayers is not limited to the foregoing adjustment.

The negative-electrode active material particles P2 contained in thesecond layer 52 b preferably have a smaller BET specific surface areathan the negative-electrode active material particles P1 contained inthe first layer 52 a. This facilitates the formation of thenegative-electrode active material layer 52 in which the second layer 52b has a higher compression modulus than the first layer 52 a. Thenegative-electrode active material particles P2 preferably have a BETspecific surface area of, for example, 0.5 m²/g to smaller than 3.5 m²/gand more preferably have a BET specific surface area of, for example,0.75 m²/g to 1.9 m²/g. The negative-electrode active material particlesP1 preferably have a BET specific surface area of, for example, 3.5 m²/gto 5 m²/g and more preferably have a BET specific surface area of, forexample, 2.5 m²/g to 4.5 m²/g. A BET specific surface area is measuredby using a conventionally known specific-surface-area measuring device(e.g., Macsorb (registered trademark) HM model-1201 by Mountech Co.Ltd.) according to the BET method.

Any negative-electrode active material particles may be selectedaccording to the type of the electrical storage device 10. For example,in the case of lithium-ion secondary batteries, materials capable ofreversibly occluding and releasing lithium ions may be used.Specifically, the materials may be carbon materials such as naturalgraphite, artificial graphite, non-graphitizable carbon, andgraphitizable carbon, surface-modified carbon materials that are carbonmaterials covered with amorphous carbon films, metals alloyed withlithium, e.g., silicon (Si) and tin (Sn), or alloys or composite oxidescontaining metallic elements such as Si and Sn. At least one of thematerials may be used alone or in combination. The negative-electrodeactive material particles P2 contained in the second layer 52 b containthe surface-modified carbon materials. An amorphous carbon filmpreferably accounts for at least 1.5 mass % relative to asurface-modified carbon material. Thus, the second layer 52 b is easilyformed with a high compression modulus. The negative-electrode activematerial particles P1 contained in the first layer 52 a contain thesurface-modified carbon materials. The content of an amorphous carbonfilm is preferably lower than 1.5 mass % relative to a surface-modifiedcarbon material. Thus, the first layer 52 a is easily formed with a lowcompression modulus.

The negative-electrode active material particles P1 contained in thefirst layer 52 a preferably have pores therein with a porosity rate of10% or more. Thus, the first layer 52 a is easily formed with a lowcompression modulus. The negative-electrode active material particles P2contained in the second layer 52 b preferably have pores therein with aporosity rate of less than 10%. Thus, the second layer 52 b is easilyformed with a high compression modulus.

The second layer 52 b preferably has a higher porosity rate than thefirst layer 52 a. This may increase the penetration of an electrolyticsolution into the negative-electrode active material layer 52 andcontribute to, for example, suppression of an increase in resistanceduring high-rate charge and discharge or suppression of a reduction incapacity in a charge and discharge cycle.

In this case, a particle-internal porosity rate for thenegative-electrode active material particles P1 and P2 is atwo-dimensional value determined from the area ratio of the internalpores of the negative-electrode active material particles relative tothe cross-sectional area of the negative-electrode active materialparticles. The porosity rates of the first layer 52 a and the secondlayer 52 b are two-dimensional values, each being determined from thearea ratio of pores between particles in each layer relative to thecross-sectional area of each layer. For example, the porosity rates aredetermined by the following steps:

-   -   (1) The negative electrode is partially cut, and then the        negative electrode is processed by an ion milling device (e.g.,        IM4000 by Hitachi High-Tech Corporation) so as to expose the        cross section of the negative-electrode active material layer        52.    -   (2) A reflection electron image of the cross section of the        first layer 52 a in the exposed negative-electrode active        material layer 52 is captured by using a scanning electron        microscope.    -   (3) The cross-sectional image thus obtained is captured by a        computer and is binarized using image analysis software (e.g.,        ImageJ by the National Institutes of Health), obtaining a binary        image in which the cross sections of particles in the        cross-sectional image are black and pores between particles and        pores in particles are white.    -   (4-1) When the particle-internal porosity rate of the        negative-electrode active material particles P1 is determined,        particles having a particle size of 5 μm to 50 μm are selected        from the binary image, and then the area of the cross sections        of the particles and the area of internal pores in the cross        sections of the particles are calculated. In this case, the area        of the cross sections of the particles means the area of regions        surrounded by the outer edges of the particles; that is, the        area of all the cross sections of the particles. From the        calculated area of the cross sections of the particles and the        calculated area of internal pores in the cross sections of the        particles, a particle-internal porosity rate (the area of        internal pores in the cross sections of the particles×100/the        area of the cross sections of the particles) is calculated. The        particle-internal porosity rate is the mean value of ten        particles.    -   (4-2) When the porosity rate of the first layer 52 a is        determined, the area of pores between particles in a measurement        range (50 μm×50 μm) is calculated from a binary image. The        cross-sectional area (2500 μm²=50 μm×50 μm) of the first layer        52 a is set as the measurement range, and the porosity rate of        the first layer 52 a (the area of pores between        particles×100/the cross-sectional area of the negative-electrode        active material layer 52) is calculated from the calculated area        of pores between particles.

The particle-internal porosity rate of the negative-electrode activematerial particles P2 and the porosity rate of the second layer 52 b aresimilarly measured.

A method of adjusting the porosity rates of the first layer 52 a and thesecond layer 52 b is, for example, a method of adjusting roll forcesapplied to the first coating and the second coating during the formationof the negative-electrode active material layer 52. Theparticle-internal porosity rate of the negative-electrode activematerial particles is adjusted in the process of manufacturing thenegative-electrode active material particles.

The negative-electrode active material particles P2 contained in thesecond layer 52 b are preferably hard particles having a 10% proofstress of 5 MPa or more. The 10% proof stress means a pressure when thenegative-electrode active material particles are compressed by a volumeratio of 10%. The 10% proof stress of a negative-electrode activematerial particle can be measured by using, for example, a microcompression tester (MCT-211 by SHIMADZU CORPORATION). For example, thenegative-electrode active material particles P1 contained in the firstlayer 52 a are preferably particles softer than the negative-electrodeactive material particles P2 and have a 10% proof stress of 3 MPa orless.

The compression modulus of the second layer 52 b is preferably at least1.2 times and more preferably at least twice as high as the compressionmodulus of the first layer 52 b. The satisfaction of the range maysuppress, for example, an increase in resistance during high-rate chargeand discharge or suppress a reduction in capacity in a charge anddischarge cycle.

The negative-electrode active material layer 52 has a thickness of, forexample, 40 μm to 120 μm, preferably 50 μm to 90 μm on one side of thenegative-electrode current collector 50. The thickness of thenegative-electrode active material layer 52 is measured from across-sectional image of the negative electrode 38 b, the image beingcaptured by a scanning electron microscope (SEM).

For the negative-electrode current collector 50, a metal leaf that isstable in the potential range of the negative electrode 38 b or a filmhaving a metallic surface layer is used. For example, materials such ascopper are used for lithium-ion secondary batteries.

The positive electrode 38 a includes, for example, a positive-electrodecurrent collector, and a positive-electrode active material layer formedon the positive-electrode current collector. For the positive-electrodecurrent collector, a metal leaf that is stable in the potential range ofthe positive electrode or a film having a metallic surface layer isused. For example, materials such as aluminum and an aluminum alloy areused for lithium-ion secondary batteries. The positive-electrode activematerial preferably contains positive-electrode active materialparticles, a conductive material, and a binder and is preferablyprovided on both sides of the positive-electrode current collector. Thepositive electrode 38 a can be produced by, for example, applying to thepositive-electrode current collector a coating of positive-electrodemixture shiny containing a positive-electrode active material, aconductive material, and a binder, drying the coating, and thencompressing the coating into a positive-electrode active material layeron each side of the positive-electrode current collector.

Any positive-electrode active material may be selected according to thetype of the electrical storage device 10. For example, in the case oflithium-ion secondary batteries, a lithium-transition metal compositeoxide is used. A lithium-transition metal composite oxide containsmetallic elements such as Ni, Co, Mn, Al, B, Mg, Ti, V. Cr, Fe, Cu, Zn,Ga, Sr, Zr, Nb, In, Sn, Ta, and W. From among the metallic elements, atleast one of Ni, Co, and Mn is preferably contained. A preferredcomposite oxide is, for example, a lithium-transition metal compositeoxide containing Ni. Co, and Mn, or a lithium-transition metal compositeoxide containing Ni, Co, and Al.

The separator 38 d is, for example, a porous sheet with ion permeationand insulation. Specific examples of a porous sheet include amicroporous thin film, a woven fabric, and a nonwoven fabric. Theseparator 38 d is preferably made of materials including olefin resinssuch as polyethylene and polypropylene and cellulose. The separator 38 dmay be a stack including a cellulose fiber layer and athermoplastic-resin fiber layer containing olefin resin. Alternatively,the separator 38 d may be a multilayer separator including apolyethylene layer and a polypropylene layer. The surface of theseparator 38 d may be coated with materials such as aramid resin andceramic.

The separator 38 d may have any compression modulus lower than that ofthe first layer 52 a of the negative-electrode active material layer 52.For example, the compression modulus is preferably 0.3 to 0.7 times,more preferably 0.4 to 0.6 times the compression modulus of the firstlayer 52 a, in view of effective suppression of an increase inresistance during high-rate charge and discharge or a reduction incapacity during a charge and discharge cycle. The compression modulus ofthe separator 38 d is adjusted by controlling, for example, theselection of materials, a porosity rate, and a pore size.

Any electrolytic solution may be selected according to the type of theelectrical storage device 10. For example, in the case of lithium-ionsecondary batteries, a nonaqueous electrolytic solution containing asupporting electrolyte in an organic solvent (nonaqueous solvent) isused. The nonaqueous solvent may be a solvent containing, for example,esters, ethers, nitriles, or amides, or a mixed solvent containing atleast two kinds of these compounds. The supporting electrolyte is, forexample, a lithium salt such as LiPF₆.

The materials of the elastic body 40 include, for example, thermosettingelastomers such as natural rubber, polyurethane rubber, silicone rubber,and fluorocarbon rubber, and thermoplastic elastomers such aspolystyrene, olefin, urethane, polyester, and polyamide. These materialsmay be foamed materials. Moreover, a heat insulating material thatsupports porous materials such as silica xerogel may be used.

The elastic body 40 may have any compression modulus lower than that ofthe separator 38 d. For example, the compression modulus is preferably120 MPa or less and is more preferably 80 MPa or less, in view ofeffective suppression of an increase in resistance during high-ratecharge and discharge or a reduction in capacity during a charge anddischarge cycle.

The elastic body 40 may have a constant compression modulus in one planebut may vary in deformability in the plane as will be described below.

FIG. 7 is a schematic perspective view illustrating an example of theelastic body. The elastic body 40 in FIG. 7 has a soft portion 44 andhard portions 42. The hard portions 42 are placed on the outer edges ofthe elastic body 40 with respect to the soft portion 44. The elasticbody 40 in FIG. 7 has a structure in which the hard portions 42 aredisposed on both ends in the second direction Y and the soft portion 44is disposed between the two hard portions 42. The soft portion 44 ispreferably disposed so as to overlap the center of the long lateralfaces of the housing 13 and the center of the electrode body 38 in thefirst direction X. The hard portions 42 are preferably disposed so as tooverlap the outer edges of the long lateral faces of the housing 13 andthe outer edge of the electrode body 38 in the first direction X.

As described above, the electrical storage device 10 is expanded mainlyby the expansion of the electrode body 38. The expansion of theelectrode body 38 increases toward the center. Specifically, thedisplacement of the electrode body 38 increases toward the center in thefirst direction X and decreases from the center toward the outer edge.According to the displacement of the electrode body 38, the displacementof the electrical storage device 10 increases toward the center of thelong lateral face of the housing 13 in the first direction X anddecreases from the center toward the outer edge of the long lateral faceof the housing 13. Thus, if the elastic body 40 in FIG. 7 is placed inthe housing 13, the elastic body 40 can receive, with the soft portion44, a large load generated by a large displacement of the electrode body38 and receive, with the hard portions 42, a small load generated by asmall displacement of the electrode body 38. If the elastic body 40 inFIG. 7 is placed outside the housing 13, the elastic body 40 canreceive, with the soft portion 44, a large load generated by a largedisplacement of the electrical storage device 10 and receive, with thehard portions 42, a small load generated by a small displacement of theelectrical storage device 10.

The elastic body 40 in FIG. 7 has a recessed portion 46 in the firstdirection X. A non-recessed portion adjacent to the recessed portion 46can be partially displaced to the recessed portion 46 when receiving aload from the electrical storage device 10 or the electrode body 38.Thus, the provision of the recessed portion 46 can facilitatedeformation of the non-recessed portion. In this configuration, in orderto make the soft portion 44 more deformable than the hard portions 42,the area ratio of the recessed portion 46 to the area of the softportion 44 is preferably higher than the area ratio of the recessedportion 46 to the area of the hard portions 42 in the first direction X.On the elastic body 40 in FIG. 7 , the recessed portion 46 is disposedonly in the soft portion 44. The recessed portion 46 may be disposed inthe hard portions 42.

The recessed portion 46 includes a core portion 46 a and a plurality oflinear portions 46 b. The core portion 46 a is shaped like a circledisposed at the center of the elastic body 40 in the first direction X.The linear portions 46 b are radially extended from the core portion 46a. Since the linear portions 46 b are radially extended, the ratio ofthe linear portions 46 b to the non-recessed portions increases towardthe core portion 46 a and decreases toward the core portion 46 a. Thus,the deformability of the non-recessed portions increases near the coreportion 46 a.

The elastic body 40 may have a plurality of through holes penetratingthe elastic body 40 in the first direction X, instead of or along withthe recessed portion 46. The through holes are not illustrated. Theprovision of the through holes can facilitate deformation ofnon-penetrating portions. Hence, in order to make the soft portion 44more deformable than the hard portions 42, the area ratio of the throughholes to the area of the soft portion 44 is preferably higher than thearea ratio of the through holes to the area of the hard portions 42 inthe first direction X.

Another example of the elastic body will be described below.

FIG. 8 is a partial schematic cross-sectional view of the elastic bodydisposed between the electrode body and the housing. The elastic body 40receives a load from the electrode body 38 in the stacking direction(first direction X) of the electrode body 38. The elastic body 40includes a substrate 42 a where the hard portions 42 havingpredetermined hardness are formed, and the soft portion 44 that issofter than the hard portion 42. The hard portion 42 is a projectingportion that projects from the substrate 42 a toward the electrode body38 and may be ruptured or plastically deformed by at least apredetermined load. The soft portion 44 is shaped like a sheet disposedbetween the substrate 42 a where the hard portions 42 are formed and theelectrode body 38 near the electrode body 38. The soft portion 44 isseparated from the electrode body 38. The soft portion 44 has throughholes 44 a at positions overlapping the hard portions 42 in the firstdirection X. The hard portion 42 is inserted into the through hole 44 a,and the tip of the hard portion 42 projects from the soft portion 44.

In response to a change of the shapes of the hard portions 42, theelastic body 40 shifts from a first state in which a load from theelectrode body 38 is received by the hard portions 42 to a second statein which the load is received by the soft portion 44. In other words, inthe elastic body 40, a load applied in the stacking direction of theelectrode body 38 by the expansion of the electrode body 38 is receivedby the hard portions 42 (first state). Thereafter, if the amount ofexpansion of the electrode body 38 increases for some reason so as toapply an excessive load to the hard portions 42, the hard portions 42are ruptured or plastically deformed, the electrode body 38 comes intocontact with the soft portion 44, and the load in the stacking directionof the electrode body 38 is received by the soft portion 44 (secondstate).

EXAMPLES

The present disclosure will be further described in accordance withexamples. The present disclosure is not limited to the examples.

Example 1

[Production of a Positive Electrode]

A lithium-transition metal composite oxide expressed by the generalformula LiNi_(0.82)Co_(0.15)Al_(0.03)O₂ was used as a positive-electrodeactive material. The positive-electrode active material, acetyleneblack, and polyvinylidene fluoride were mixed at a solid mass ratio of97:2:1, and positive-electrode mixture slurry was prepared by usingN-methyl-2-pyrrolidone (NMP) as a dispersion medium. Subsequently, acoating of the positive-electrode mixture slurry was applied to eachside of a positive-electrode current collector composed of aluminumfoil. The coating was dried, rolled, and then cut into a predeterminedelectrode size, so that a positive electrode was obtained with apositive-electrode active material layer formed on each side of thepositive-electrode current collector.

[Preparation of the First Negative-Electrode Mixture Slurry]

On the surfaces of graphite particles, a surface-modified carbonmaterial A coated with an amorphous carbon film of 0.6 mass % was usedas a negative-electrode active material. The surface-modified carbonmaterial A has a BET specific surface area of 3.9 m²/g (see otherphysical properties in Table 1). The surface-modified carbon material A,the dispersion of styrene-butadiene rubber (SBR), and sodiumcarboxymethylcellulose (CMC-Na) were mixed at a solid mass ratio of100:1:1, and the first negative-electrode mixture slurry was prepared byusing water as a dispersion medium.

[Preparation of the Second Negative-Electrode Mixture Slurry]

On the surfaces of graphite particles, a surface-modified carbonmaterial B coated with an amorphous carbon film of 1.5 mass % was usedas a negative-electrode active material. The surface-modified carbonmaterial B had a BET specific surface area of 2.9 m²/g (see otherphysical properties in Table 1). The surface-modified carbon material B,the dispersion of SBR, and CMC-Na were mixed at a solid mass ratio of100:1:1, and the second negative-electrode mixture slurry was preparedby using water as a dispersion medium.

[Production of a Negative Electrode]

A coating of the first negative-electrode mixture slurry was applied toeach side of the negative-electrode current collector composed of copperfoil, the coating was dried and rolled, a coating of the secondnegative-electrode mixture slurry was applied onto the coating, and thecoating was dried and rolled, so that a negative-electrode activematerial layer was formed with a first layer of the firstnegative-electrode mixture slurry, and a second layer of the secondnegative-electrode mixture slurry on the negative-electrode currentcollector. The negative-electrode active material layer was cut into apredetermined electrode size so as to obtain a negative electrode. Thenegative-electrode active material layer having a thickness of 160 μm(except for the negative-electrode current collector) was formed withapplication of equal amounts of the first and second negative-electrodemixture slurry. During the production of the negative electrode, themeasured compression moduli of the first and second layers were 660 MPaand 830 MPa (see other physical properties in Table 1).

[Preparation of an Electrolytic Solution]

Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethylcarbonate (DMC) were mixed at a volume ratio of 3:3:4. An electrolyticsolution was prepared by dissolving LiPF₆ with a concentration of 1.4mol/L in the mixed solvent.

[Production of the Electrical Storage Device]

Negative electrodes, separators with a compression modulus of 230 MPa,and positive electrodes were sequentially stacked to produce anelectrode body. The negative electrodes and the positive electrodes wereconnected to the positive terminal and the negative terminal, which hadbeen attached to the sealing plate, and then were inserted with theelectrolytic solution into a rectangular outer can. The opening of theouter can was sealed with the sealing plate, completing the productionof the electrical storage device.

[Production of the Electrical Storage Module]

The produced electrical storage device was held by a pair of elasticbodies (a urethane foam sheet having a compression modulus of 120 MPa)to oppose the long lateral face and was held and fixed by a pair of endplates, so that the electrical storage module was produced. The elasticbody is disposed so as to receive a load from the electrode body in thestacking direction of the electrode body when the electrode bodyexpands.

Example 2

The electrical storage module was produced as in Example 1 except thatthe separator with a compression modulus of 120 MPa was used and theelastic body was a urethane foam sheet having a compression modulus of60 MPa.

Example 3

The electrical storage module was produced as in Example 1 except thatthe separator had a compression modulus of 120 MPa and the elastic bodywas a pair of nitrile rubber sheets having a compression modulus of 5MPa.

Example 4

The electrical storage module was produced as in Example 1 except thatthe separator with a compression modulus of 80 MPa was used and theelastic body was a pair of urethane foam sheets having a compressionmodulus of 40 MPa.

Example 5

The electrical storage module was produced as in Example 2 except thatthe negative-electrode active material was a surface-modified carbonmaterial C having a BET specific surface area of 1 m²/g and coated withan amorphous carbon film of 1.5 mass % on the surfaces of graphiteparticles in the preparation of the second negative-electrode mixtureslurry. During the production of the negative electrode, the measuredcompression moduli of the first and second layers were 660 MPa and 1200MPa.

Example 6

The electrical storage module was produced as in Example 2 except thatthe negative-electrode active material was a surface-modified carbonmaterial D having a BET specific surface area of 0.9 m²/g and coatedwith an amorphous carbon film of 3.0 mass % on the surfaces of graphiteparticles in the preparation of the second negative-electrode mixtureshiny. During the production of the negative electrode, the measuredcompression moduli of the first and second layers were 660 MPa and 1250MPa.

Example 7

The electrical storage module was produced as in Example 2 except thatthe negative-electrode active material was a surface-modified carbonmaterial E having a BET specific surface area of 4.4 m²/g and coatedwith an amorphous carbon film of 0.6 mass % on the surfaces of graphiteparticles in the preparation of the first negative-electrode mixtureshiny. During the production of the negative electrode, the measuredcompression moduli of the first and second layers were 420 MPa and 830MPa.

Example 8

The electrical storage module was produced as in Example 2 except thatthe negative-electrode active material was a surface-modified carbonmaterial E in the preparation of the first negative-electrode mixtureslurry, a roll force applied to a coating of the firstnegative-electrode mixture slurry was increased by 0.75 times, and aroll force applied to a coating of the second negative-electrode mixtureslurry was increased by 0.7 times in the production of the negativeelectrode. During the production of the negative electrode, the measuredcompression moduli of the first and second layers were 250 MPa and 620MPa.

Example 9

The electrical storage module was produced as in Example 2 except thatthe negative-electrode active material was the surface-modified carbonmaterial A in the preparation of the first negative-electrode mixtureslurry, a roll force applied to a coating of the firstnegative-electrode mixture slurry was increased by 0.8 times, and thenegative-electrode active material was a surface-modified carbonmaterial F having a BET specific surface area of 3.5 m²/g and coatedwith an amorphous carbon film of 2.0 mass % on the surfaces of graphiteparticles in the preparation of the second negative-electrode mixtureslurry. During the production of the negative electrode, the measuredcompression moduli of the first and second layers were 900 MPa and 320MPa.

Comparative Example 1

A coating of the second negative-electrode mixture slurry of Example 1was applied to each side of the negative-electrode current collectorcomposed of copper foil, and then the coating was dried and rolled, sothat a negative-electrode active material layer of the secondnegative-electrode mixture slurry was formed on the negative-electrodecurrent collector. The negative-electrode active material layer was cutinto a predetermined electrode size so as to obtain a negativeelectrode. The electrical storage module was produced as in Example 1except that the negative electrode was used and the elastic body was apair of polyethlene terephthalate sheets having a compression modulus of2800 MPa. During the production of the negative electrode, the measuredcompression modulus of the negative-electrode active material layer was830 MPa.

Comparative Example 2

The electrical storage module was produced as in Comparative Example 1except that the negative-electrode active material was thesurface-modified carbon material A in the preparation of thenegative-electrode mixture slurry, and a roll force applied to a coatingof the negative-electrode mixture slurry was increased by 0.6 times inthe production of the negative electrode. During the production of thenegative electrode, the measured compression modulus of thenegative-electrode active material layer was 200 MPa.

Comparative Example 3

The electrical storage module was produced as in Comparative Example 1except that the elastic body was a urethane foam sheet having acompression modulus of 120 MPa.

[Measurement of an Initial Resistance (IV Resistance)]

The initial resistances of the electrical storage devices according tothe examples and the comparative examples were measured under thefollowing conditions: The electrical storage device adjusted in acharging state of SOC 60% was subjected to constant current discharge ata temperature of 25° C. at 5 C rate for ten seconds, so that a voltagedrop (V) was calculated. The value (V) of the voltage drop was dividedby a corresponding current value (I) to calculate an IV resistance (mΩ),and then the mean value of the resistance was determined as an initialresistance.

[High-Rate Charge and Discharge Test]

Subsequently, for the electrical storage modules according to theexamples and the comparative examples, a charge and discharge cycle testwas conducted such that ten cycles of charge and discharge were repeatedat a temperature of 25° C. After the cycle test, a rate of increase inbattery resistance was measured. In one cycle of the charge anddischarge cycle test, constant current charge was performed at a chargerate of 1.5 C for 4300 seconds, the charge was suspended for tenseconds, constant current discharge was performed at a discharge rate of1.5 C for 4300 seconds, and then the discharge was suspended for tenseconds. Subsequently, the resistances (IV resistances) of theelectrical storage modules after the charge and discharge cycle testwere measured by the same method as the measurement of the initialresistance, and a rate of increase in battery resistance was measured.The test was repeated until the rate of increase reached 200%. Thelarger the number of cycles until the rate of increase reaches 200%, thesmaller the increase in resistance during high-rate charge anddischarge.

[Measurement of a Capacity Maintenance Rate in Charge and DischargeCycle Characteristics]

For the electrical storage modules of the examples and the comparativeexamples, constant current charge was performed until a battery voltagereached 4.2 V at a temperature of 25° C. at a constant current of 0.33C, and then constant current discharge was performed until the batteryvoltage reached 3.0 V at a constant current of 0.33 C. The charge anddischarge cycle was performed 500 times, and capacity maintenance ratesin the charge and discharge cycles of the electrical storage moduleswere determined by the formula below. The higher the capacitymaintenance rate, the smaller the reduction in capacity in the chargeand discharge cycle.Capacity maintenance rate=(discharged capacity in the 500-thcycle/discharged capacity in the first cycle)×100

Table 1 shows the physical properties of the negative-electrode activematerial layers (the first layer and the second layer) of the examplesand the comparative examples. Table 2 shows the compression moduli ofthe negative-electrode active material layers (the first layer and thesecond layer), the compression moduli of the separator, and thecompression moduli of the elastic body according to the examples and thecomparative examples, and the test results of the examples and thecomparative examples.

TABLE 1 SECOND LAYER FIRST LAYER NEGATIVE-ELECTRODE NEGATIVE-ELECTRODEACTIVE MATERIAL ACTIVE MATERIAL AMOR- COM- AMOR- COM- PHOUS PRES- PHOUSPRES- CARBON 10% PARTICLE- PO- SION CARBON 10% PARTICLE- PO- SION FILMPROOF INTERNAL ROSITY MOD- FILM PROOF INTERNAL ROSITY MOD- BET AMOUNTSTRESS POROSITY RATE ULUS BET AMOUNT STRESS POROSITY RATE ULUS (m²/g)(wt. %) (MPa) RATE (%) (%) (MPa) (m²/g) (wt. %) (MPa) RATE (%) (%) (MPa)EXAMPLE 1 2.9 1.5 5.7 5 24 830 3.9 0.6 2.1 10 22 660 EXAMPLE 2 2.9 1.55.7 5 24 830 3.9 0.6 2.1 10 22 660 EXAMPLE 3 2.9 1.5 5.7 5 24 830 3.90.6 2.1 10 22 660 EXAMPLE 4 2.9 1.5 5.7 5 24 830 3.9 0.6 2.1 10 22 660EXAMPLE 5 1 1.5 44 0 26 1200 3.9 0.6 2.1 10 22 660 EXAMPLE 6 0.9 3 27 726 1250 3.9 0.6 2.1 10 22 660 EXAMPLE 7 2.9 1.5 5.7 5 24 830 4.4 0.6 1.815 23 420 EXAMPLE 8 2.9 1.5 5.7 5 31 620 4.4 0.6 1.8 15 28 250 EXAMPLE 93.5 2 11 3 23 900 3.9 0.6 2.1 10 25 320 COMPAR- 2.9 1.5 5.7 5 24 830 — —— — — — ATIVE EXAMPLE 1 COMPAR- 3.9 0.6 2.1 10 36 200 — — — — — — ATIVEEXAMPLE 2 COMPAR- 2.9 1.5 5.7 5 24 830 — — — — — — ATIVE EXAMPLE 3

TABLE 2 SECOND ELASTIC TEST RESULT LAYER FIRST LAYER SEPARATOR BODYNUMBER OF CYCLES COMPRESSION COMPRESSION COMPRESSION COMPRESSIONCAPACITY (300% INCREASE IN MODULUS MODULUS MODULUS MODULUS MAINTENANCERESISTANCE) (MPa) (MPa) (MPa) (MPa) RATE (%) (NUMBER OF CYCLES) EXAMPLE1 830 660 230 120 84 120 EXAMPLE 2 830 660 120 60 85 170 EXAMPLE 3 830660 120 5 84 220 EXAMPLE 4 830 660 80 40 84 130 EXAMPLE 3 1200 660 12060 86 180 EXAMPLE 6 1250 660 120 60 86 190 EXAMPLE 7 830 420 120 60 89120 EXAMPLE 8 620 250 120 60 90 180 EXAMPLE 9 900 320 120 60 83 120COMPARATIVE 830 — 230 2800 75 30 EXAMPLE 1 COMPARATIVE 200 — 230 120 8250 EXAMPLE 2 COMPARATIVE 830 — 230 120 79 80 EXAMPLE 3

According to Examples 1 to 8 in which a compression modulus satisfiesthe relationship of the second layer near the surface>the first layernear the negative-electrode current collector>the separator>the elasticbody, higher capacity maintenance rates and lower rates of increase inresistance were obtained as compared with Comparative Examples 1 to 3 inwhich compression modulus does not satisfy the relationship.Furthermore. Examples 1 to 8 suppressed a reduction in capacitymaintenance rate in a charge and discharge cycle and an increase inresistance during high-rate charge and discharge.

REFERENCE SIGNS LIST

-   1 Electrical storage module-   2 Electrical storage stack-   4 End plate-   6 Locking member-   8 Cooling plate-   Electrical storage device-   12 Insulating spacer-   13 Housing-   14 Outer can-   16 Sealing plate-   18 Output terminal-   38 Electrode body-   38 a Positive electrode-   38 b Negative electrode-   38 d Separator-   39 Winding core-   40 Elastic body-   42 Hard portion-   42 a Substrate-   44 Soft portion-   44 a Through hole-   46 Recessed portion-   46 a Core portion-   46 b Linear portion-   50 Negative-electrode current collector-   52 Negative-electrode active material layer-   52 a First layer-   52 b Second layer

The invention claimed is:
 1. An electrical storage module comprising: atleast one electrical storage device, and an elastic body that is placedwith the electrical storage device and receives a load from theelectrical storage device in a placement direction of the elastic body,wherein the electrical storage device includes an electrode body inwhich a positive electrode, a negative electrode, and a separatordisposed between the positive electrode and the negative electrode arestacked, and a housing accommodating the electrode body, the negativeelectrode includes a negative-electrode current collector, and anegative-electrode active material layer that is formed on thenegative-electrode current collector and contains negative-electrodeactive material particles, the negative-electrode active material layerincluding a first layer formed on the negative-electrode currentcollector, and a second layer that is formed on the first layer and hasa higher compression modulus than the first layer, the separator has alower compression modulus than the first layer, and the elastic body hasa lower compression modulus than the separator.
 2. The electricalstorage module according to claim 1, wherein the negative-electrodeactive material particles contained in the second layer have a smallerBET specific surface area than the negative-electrode active materialparticles contained in the first layer.
 3. The electrical storage moduleaccording to claim 1, wherein the negative-electrode active materialparticles contained in the second layer contain a surface-modifiedcarbon material with a surface of a carbon material coated with anamorphous carbon film, and a content of the amorphous carbon film is atleast 1.5 mass % relative to the surface-modified carbon material. 4.The electrical storage module according to claim 1, wherein thenegative-electrode active material particles contained in the firstlayer contain a carbon material, and the carbon material has aparticle-internal porosity rate of at least 10%.
 5. The electricalstorage module according to claim 1, wherein the second layer has ahigher porosity rate than the first layer.
 6. The electrical storagemodule according to claim 1, wherein the elastic body has a compressionmodulus of at most 120 MPa.
 7. The electrical storage module accordingto claim 1, wherein the elastic body is a stack of a plurality ofelastic sheets.
 8. The electrical storage module according to claim 1,wherein the elastic body has a soft portion, and a hard portion placedon an outer edge of the elastic body with respect to the soft portion,the soft portion being more deformable than the hard portion.
 9. Theelectrical storage module according to claim 1, wherein the elastic bodyhas a hard portion, and a soft portion more deformable than the hardportion, the hard portion is deformed by the load not smaller than apredetermined load, and the elastic body shifts, in response todeformation of the hard portion, from a first state in which the load isreceived by the hard portion to a second state in which the load isreceived by the sail portion.
 10. An electrical storage devicecomprising: an electrode body in which a positive electrode, a negativeelectrode, and a separator disposed between the positive electrode andthe negative electrode are stacked, an elastic body configured toreceive a load from the electrode body in an stacking direction of theelectrode body, and a housing accommodating the electrode body and theelastic body, wherein the negative electrode includes anegative-electrode current collector, and a negative-electrode activematerial layer that is formed on the negative-electrode currentcollector and contains negative-electrode active material particles, thenegative-electrode active material layer including a first layer formedon the negative-electrode current collector, and a second layer that isformed on the first layer and has a higher compression modulus than thefirst layer, the separator has a lower compression modulus than thefirst layer, and the elastic body has a lower compression modulus thanthe separator.